1. Field of the Invention
The present invention relates to a method for measuring an internal property distribution of a measured object and an apparatus therefor. More particularly, the present invention concerns an internal property distribution measuring method and an apparatus therefor applicable to optical CT (computed tomography) apparatus or the like for obtaining a tomographic image by moving the light incidence position and light detection position along the surface of measured object.
2. Related Background Art
In the optical CT apparatus wherein measurement light is incident at one light incidence position on the surface of the object being a scattering medium, wherein the measurement light transmitted as scattered by the object is received at a plurality of light detection positions on the surface of the object, and wherein a distribution of an internal property in the scattering medium is obtained as moving the light incidence position and light detection position along the surface of the object, the following methods are known as methods for obtaining a distribution of absorption coefficient inside thereof, for example. Specifically, they are the methods described in "Imaging of Multiple Targets in Dense Scattering Media" (H. L. Graber, J. Chang, R. L. Barbour, SPIE vol. 2570, p. 219-p. 234), "Imaging diffusive media using time-independent and time-harmonic sources; dependence of image quality on imaging algorithms, target volume weight matrix, and view angles" (Jenghwa Chang et al., SPIE vol. 2389), and so on.
The basic imaging principle in such conventional methods is to use a relational equation between received light and a function indicating a contribution to the received light (which is referred to as "spread function" for convenience) where the inside of measured object is divided into a plurality of voxels for convenience, light incident from a certain point on the surface of object passes through the inside of measured object and is received at another point on the same surface, and on that occasion attention is focused on a specific internal property such as an absorption coefficient for each voxel. The voxel stated herein means each region (volume element) obtained by dividing the measured object into a plurality of regions.
In the above conventional methods, however, a phantom without absorption was prepared separately from the measured object, the quantity of detected light to be a reference was measured using it, and an aimed absorption coefficient distribution inside the scattering medium was obtained using the spread function in that state. For imaging with such methods, it was necessary to assume a phantom model (physical model) or a simulation model made so as to have a shape identical or similar to the measured object and so as to have a known internal property and to use data obtained from such a model as a reference value in calculation of imaging. Therefore, these conventional methods were not able to avoid errors caused by the difference between the actual measured object and the physical model or simulation model, individual differences of measured object, and so on, and it was very difficult to apply them, especially, to measured objects having complex structure, such as a living body.
On the other hand, a method for obtaining a spatial distribution of concentration of absorptive substance without using a phantom is the method described in the bulletin of Japanese Laid-open Patent Application No. 8-29329. The method described in the same bulletin, however, needed to use light having a plurality of wavelengths even for the cases of only one absorptive constituent in the measured object, and the spatial distribution of concentration of absorptive substance was obtained under the assumption that a mean optical pathlength distribution and attenuated light quantity (the quantity of light attenuated due to influence of scattering or the like) were constant among these wavelengths. In addition, this method assumed an imaginary subject without absorptive substance and obtained the spatial distribution of concentration of absorptive substance using the mean optical pathlength in the imaginary subject, but it did not take the change of optical pathlength due to absorption into consideration. Therefore, the method described in the above bulletin was not satisfactory yet as to reliability of internal property distribution obtained.